With the proliferation of portable electronic devices, the use of rechargeable batteries has become increasingly important. Rechargeable batteries can now be found in devices as simple as a flashlight, as important as notebook computers, and as vital as portable medical equipment. An example of a portable medical device which is dependent on a rechargeable battery pack is a portable defibrillator unit.
Portable defibrillator units are used by emergency medical technicians and others on persons suffering from certain types of abnormal heart rhythms, e.g., ventricular fibrillation, to shock the heart back into a normal beating pattern. Although many of these portable defibrillators have the ability to operate off of AC line current, when used in the field, portable defibrillators are almost totally dependent on rechargeable battery packs. The portable battery packs provide the power both to operate the internal electronics of the defibrillator and to provide the charge source for the therapeutic shock. In order to provide the power source for charging the shock delivery portion of the defibrillator, it is necessary that the portable battery pack be capable of providing a relatively large current draw over a relatively short period of time. If the battery is unable to supply this current when demanded, the delivery of a therapeutic shock may be delayed or prohibited. Therefore, it is important to maintain a state of charge in a portable battery pack that is sufficient to deliver therapeutic shocks.
Seconds count in the application of the therapeutic shock to a person suffering a heart attack. Swapping a bad battery pack in and out of a defibrillator may waste this precious time, as may waiting for a marginally functional battery to deliver the charge necessary for the therapeutic shock. It is important, therefore, for the user of a portable defibrillator to make sure that a reliable, working battery pack is available. This has usually meant having an ample supply of extra battery packs available. Unfortunately, one can usually only guess the ability of the battery pack to reliably deliver high current charging pulses. While users normally log the age and use of the battery manually to predict its current condition, the accuracy of the predictions are both dependent on the accuracy of the records and the validity of the underlying assumptions of the predictions. Therefore, it is important to provide regular, reliable maintenance and testing of portable battery packs.
In response to the demand for batteries that provide a means to determine their state of charge, computer and battery manufacturers have been recently developing "smart batteries," which internally measure battery variables such as voltage and current flow in and out of the battery and then apply predictive algorithms to estimate the battery's state of charge. The battery's predicted state of charge can then be communicated to a portable electronic device such as a notebook computer (a "host") over a communication bus. This is useful in applications where a computer needs to find out if there is enough charge left in the battery to save a word-processing file to a disk drive. However, the prediction of a smart battery's state of charge must be much more reliable in medical device equipment, such as a defibrillator, where the battery's actual ability to deliver charge is crucial to the appropriate treatment of an individual. This is particularly true if the only way to determine if the battery is able to deliver the charge is by first inserting it into the host unit. This again demonstrates the need for reliable maintenance and testing.
However, there is more to the maintenance of a rechargeable battery pack than just charging it. These additional maintenance steps are often specific to the battery chemistry. Rechargeable battery packs are currently manufactured using a number of known battery chemistries, including nickel cadmium (NiCd), sealed lead acid (SLA), nickel-metal hydride (NiMH), lithium ion (Li-ion), lithium polymer (Li-polymer), and rechargeable alkaline. The most popular choice for rechargeable batteries is currently the NiCd chemistry because it is relatively inexpensive, is fast and easy to charge, has excellent load performance even at cold temperatures, and is capable of withstanding a high number of charge/discharge cycles. Over the course of the life of the NiCd battery, however, the cycling of the battery causes it to develop crystalline formations which substantially decreases the battery's ability to hold charge. This decrease is commonly referred to as "memory." It is known that NiCd memory can be substantially reduced by "conditioning" the battery, which involves fully discharging the battery and then charging the battery back to the state of full charge. This process breaks down the crystalline structure developed over time and enables the battery to receive and store a greater charge.
If the NiCd "memory" goes undetected, it may show a voltage indicating a full charge while it actually does not hold sufficient charge to supply the high current pulse required by a demanding application such as a portable defibrillator. While this "memory" problem has long been recognized, the conditioning required to correct it has depended on the user manually conditioning the battery on a regular basis. This meant that the user had to estimate when the battery required conditioning and then manually put the battery through a conditioning process. The actual discharge and charging of the battery during conditioning can take hours during which the battery is out of service. Consequently, rechargeable battery packs are sometimes used past the period in which they should be reconditioned, used until they fail, or are simply discarded much earlier than they would actually need to be if they were properly maintained.
Another cause of battery failure is that rechargeable batteries experience self-discharge. The amount of this self-discharge varies according to battery chemistry, age, or the presence of manufacturing defects. For instance, according to one source, the NiCd loses about 10% of its capacity within the first 24 hours, after which the self-discharge settles to about 10% per month. The rate of this self-discharge generally increases as the battery ages. Therefore, the battery is constantly losing power whether it is being used or not. If the self-discharge becomes excessive, it may quickly lose its ability to reliably deliver its charge while sitting on the shelf waiting for use.
Accordingly, a method and apparatus for reliably maintaining and testing a portable battery pack are needed. The method and apparatus should accurately and consistently predict the state of charge of the portable battery. Further, the method and apparatus should automatically and efficiently recondition the battery, based on the battery's chemistry, as soon as the need for conditioning is detected. The method and apparatus should also be able to measure the self-discharge of a battery being tested. As explained in the following, the present invention provides a method and apparatus that meets these criteria and solves other problems in the prior art.